1. Field of the Invention
The present invention relates to an injector, which injects and supplies fuel into an engine.
2. Description of Related Art
An injector, which injects fuel directly into an combustion chamber of an engine, conventionally has the following configuration. As shown in FIG. 6A to FIG. 6C, a conventional injector 100 includes a valve body 102, which defines a internal space 101 having a generally cylindrical shape, and a needle valve 103, which is received in the internal space 101 and moves in its axial direction.
The internal space 101 is covered with a bottom 105 of the valve body 102, through which a nozzle hole 104 penetrates. The valve body 102 accommodates the needle valve 103 in the internal space 101. Accordingly, a fuel passage 106 leading into a nozzle hole 104 is defined between an inner circumferential surface of the valve body 102 and an outer circumferential surfaces of the needle valve 103. The injector 100 starts or stops injection of fuel by opening or closing the fuel passage 106 with respect to the nozzle hole 104 as a result of the movement of the needle valve 103.
According to the injector 100, even though the fuel passage 106 is closed with respect to the nozzle hole 104 by the needle valve 103, an internal space (hereinafter referred to as a bottom chamber 108) under a bottom face 107 of the needle valve 103 always communicates with a combustion chamber through the nozzle hole 104. Consequently, high-temperature combustion gas enters into the bottom chamber 108 via the nozzle hole 104, thereby carbonizing residual fuel on the bottom face 107 (see FIG. 6B). As a result, deposits accumulate on the bottom face 107 so as to hinder the injection of fuel, and thus spray characteristics of the injector 100 may fluctuate (see FIG. 6C).
According to conventional technologies, by applying a coating of a fluorine system to the whole bottom-side surface of the needle valve 103 including the bottom face 107, exfoliation of fuel from the bottom face 107 is promoted, so that fuel does not remain on the bottom face 107. However, fuel cannot completely be prevented from remaining on the bottom face 107. Therefore, the problem that residual fuel is carbonized to become deposits cannot be fully resolved, so measures against the accumulation of deposits need to be separately taken.
According to a technology described in JP2006-329147A, a coating of a fluorine system or the like is applied in a streaked manner to a certain area of a wall surface of a nozzle hole, and then deposits accumulated on the wall surface of the nozzle hole are sheared off to be removed using a difference in a coefficient of thermal expansion between a coated area and an uncoated area of the wall surface. Nevertheless, it is extremely complicated to apply a coating only to the area of one surface where the coating is needed, separately from the area where the coating is not needed. Moreover, shear force in the coated area is not very strong, and therefore it is unknown whether an effect, which is worth the complicated coating operations, is produced.